Occupational Exposure to 4, 4’-Bipyridyl Vapor and Aerosol During Paraquat Manufacturing

The mass concentrations of both vapor and aerosol phases of 4,4’-bipyridyl were determined by a glass fiber filter and an XAD-2 tube in a paraquat factory. The size distributions of 4,4’-bipyridyl aerosols and particles <10 μm were measured by a six-stage cascade impactor. The mass concentrations of 4,4’ -bipyridyl aerosols were in the range of 81.9 to 159.9 mg/m³ with an average 4,4’ -bipyridyl vapor mass concentration of 3.1 mg/m³ at the top of the open tank. On the average, the mass ratio of vapor phase to aerosol phase of 4,4’ -bipyridyl was 0.025. Therefore, the aerosol phase is the major form of 4,4’ -bipyridyl exposure to the workers in this environment. From the 4,4’-bipyridyl concentrations measured by the sampling systems of an XAD-2 tube alone and a filter followed by an XAD-2 tube, the 4,4’-bipyridyl aerosol collection efficiency of the XAD-2 tube was found to be approximately 50%. The average mass concentrations of less than 10 μm aerosols and 4,4’ -bipyridyl aerosol in the occupational environment were 0.42 mg/m³ and 1.26 μg/m³, respectively. The mass median aerodynamic diameter (geometric standard deviation) of particles <10 μm and 4,4’-bipyridyl aerosol are 3.50 μm (2.35) and 3.47 μm (2.58), respectively. The major mass fraction of 4,4’-bipyridyl aerosol occurred in the 33-5.8 μm size range. The enclosure efficiencies of a hood cover attached to a suction tube were found to be 88% and 82% for both aerosol and vapor phases of 4,4’ -bipyridyl, respectively. It is suggested that the heterogeneous nucleation of 4,4’-bipyridyl and water vapor from the open tanks with subsequent coagulation is the major mechanism of aerosol formation. We conclude that both 4,4’-bipyridyl aerosol and vapor concentration should be determined for the exposure assessment of the paraquat manufacturer in this workplace.